(a). Field of the Invention
The present invention relates to a coated disk substrate having a small thickness region and, more particularly, to an improved disk substrate capable of improving optical characteristics of optical storage medium.
(b). Description of the Related Art
Disk substrates for use in fabrication of optical storage media or optical disk are generally made by injecting a synthesized resin into a molding die assembly such as shown in FIG. 10 to have an annular shape.
The molding die assembly of FIG. 10 comprises a stationary die 5 and a movable die 6 coupled or clamped together to form therebetween a disk-shaped cavity 9. The stationary die 5 comprises a substantially cylindrical mirror plate 50 (first mirror plate) mounted on a die body not shown in the figure, and a sprue bush 54 inserted in the central cavity of the cylindrical mirror plate 50 and locked therein by way of a locking bush 52. The movable die 6 comprises a substantially cylindrical mirror plate (second mirror plate) 60 mounted on a movable die body not shown, a cylindrical inner stamper holder 65 disposed in the central cavity of the mirror plate 60 for holding the inner edge of a stamper, and a cut-out punch 64 inserted in the central cavity of the inner stamper holder 65 and locked therein by way of a locking bush 62.
In operation of the molding die assembly, an annular stamper 7 is disposed on the inner surface of the second mirror plate 60, and is fixed at the inner edge thereof by the inner stamper holder 65 and fixed at the outer edge thereof by a cavity outer ring 8 disposed on the outer surface of the first mirror plate 50.
Molten resin is injected into the cavity 9 through a resin path 55 formed in the sprue bush 54, and compression-molded in the cavity 9 to form a disk substrate. In the molding step, the stamper 7 provides the geometry of the recording area of the disk substrate having a pattern for pits and grooves, whereas the cut-out punch 64 provides a central opening of the disk substrate.
FIG. 11 shows the cross-section of a disk substrate manufactured by the molding die assembly of FIG. 10. The annular disk substrate has a recording area 2 and a non-recording area 3 disposed consecutively from the outer periphery toward the central opening of the disk substrate on one side (recording side) of the disk substrate. Another non-recording area 1 may be also disposed in the vicinity of the outer periphery, as shown in the figure.
In FIG. 11, a dent 3c is formed adjacent to the outer periphery of the inner non-recording area 3, the dent 3c having a cross-section either trapezoid, oblong, semicircle or semi-ellipse depending on the geometry of the molding die. The dent 3c is formed by the rim 6c of the inner stamper holder 65 protruding therefrom for holding the stamper 7. The dent 3c is subjected to a stress concentration due to an external force during a sputtering process for depositing a recording layer or protective layer or after mounting the disk on a driving hub, the stress concentration causing a warp in the disk substrate. The warp may reduce the servo characteristics of the resultant optical disk due to a local optical strain or inclination of the recording surface of the disk.